forced induction piston rings

[deth2u]

I read in my auto book in class the chapter on forced induction. The interesting part was when they skimmed over the piston rings. They insinuated that you need piston rings with a smaller clearance between the piston and block walls for the increased cylinder pressure. Or do I have it backwards? Doesn't this make for increased blowby and loss of power? Or under extreme pressure (overboost) it'll blow the rings right off? Can anyone shed some light here?

 

[LaserRST]

Well there is a minimum clearance between pistons and cylinder walls as well as service points. On a well built engine they take into factor the amount the pistons expand when heated. This is one of the advantages of forged over cast. Forged pistons will expand less in higher temps than cast. This makes for a piston that will actually run better when run in extreme conditions. As far as piston rings there are two places where some clearances can be finely tuned to maintain good pressure within heat ranges. That is the gap and the clearance between the ring land. I spoke to some one about the pistons I bought for my car and he metioned the importance for tight clearances between the ring land for forced induction. When the piston is on its downward force it actually shifts at an angle against the cylinderwalls. This creates resistance through friction as well as heat and expansion seen mostly in cast rings. Some rings have ridges to help eliminate this in the upward force to minimize friction when it is not neccessary to hold compression. Chromoly rings dont expand as much as cast iron, gapless rings reduces the space in the gap. Proper break in will polish the rings to the cylinder walls and increase compression life. Any way in high performance pistons actually have thicker rings with the ability to have lower ring land tolerances in the top ring. This is due to the low expansion rate of the pistons, add this to chromoly rings and tight clearances = a stable piston through your entire rpm range. In overboosting you would incinerate the piston and the rings before they blow off because of high boost pressures. My .02 cents....

 

[deth2u]

so the princliple behind performance rings is to get the ring as tight against the wall as possible... but having that extra few thousandths on the rings for a better gap so to say does that make it "flimsy" or more prone to shift angles like you said?

that sounds like it would leakdown faster than a stock cast ring

 

[LaserRST]

Well for performance sake reduce the gap with tighter tolerances. Tighter tolerances will actually reduce the angle formed on the down stroke. This is why when you buy forged pistons they actually have larger top rings built for them. The larger rings reduce the space between the ring lands. This increases the stability of the entire set up.

 

[deth2u]

kinda makes sense now

thanks laser